Method for operating a high pressure electrochemical cell

ABSTRACT

The present invention relates to a unique electrochemical cell stack which employs an electrically conductive pressure pad. The pressure pad is composed of material compatible with the electrochemical cell environment and is disposed on the high pressure side of the membrane assembly, in intimate contact with the high pressure side screen pack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Application Serial No. 60/114,559 filed Dec. 31, 1998, theentire contents of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a means of maintaining compressionwithin the active area of an electrochemical cell, and especiallyrelates to the use of a pressure pad assembly to maintain compressionwithin the active area on the high pressure side of an electrochemicalcell.

BACKGROUND OF THE INVENTION

Electrochemical cells are energy conversion devices, usually classifiedas either electrolysis cells or fuel cells. A proton exchange membraneelectrolysis cell functions as a hydrogen generator by electrolyticallydecomposing water to produce hydrogen and oxygen gases. Referring toFIG. 1, in a typical single anode feed water electrolysis cell 101,process water 102 is reacted at oxygen electrode (anode) 103 to formoxygen gas 104, electrons, and hydrogen ions (protons) 105. The reactionis created by the positive terminal of a power source 106 electricallyconnected to anode 103 and the negative terminal of a power source 106connected to hydrogen electrode (cathode) 107. The oxygen gas 104 and aportion of the process water 102′ exit cell 101, while protons 105 andwater 102″ migrate across proton exchange membrane 108 to cathode 107where hydrogen gas 109, is formed.

The typical electrochemical cell includes a number of individual cellsarranged in a stack with fluid, typically water, forced through thecells at high pressures. The cells within the stack are sequentiallyarranged including a cathode, a proton exchange membrane, and an anode.The cathode/membrane/anode assemblies (hereinafter “membrane assembly”)are supported on either side by packs of screen or expanded metal whichare in turn surrounded by cell frames and separator plates to formreaction chambers and to seal fluids therein. The screen packs establishflow fields within the reaction chambers to facilitate fluid movementand membrane hydration, and to provide mechanical support for themembrane and a means of transporting electrons to and from electrodes.

In order to maintain uniform compression in the cell active area, i.e.,the electrodes, thereby maintaining intimate contact between flow fieldsand cell electrodes over long time periods, pressure pads havetraditionally been used within electrochemical cells. Pressure pads havetraditionally been fabricated from materials incompatible with systemsfluids and/or the cell membrane, such as silicone rubber, therebyrequiring that these pressure pads be disposed within a protectiveencasing.

Pressure pads are typically preloaded to stress levels which counteractthose resulting from the pressurization levels of the working fluids ofthe electrochemical cell plus approximately 50 p.s.i. to guaranteecontact between the parts. For example, in an electrolyzer whichoperates at about 400 p.s.i., the pressure pad is designed to handle 650p.s.i., which constitutes the proof pressure of the unit (1.5 times theworking pressure) plus 50 p.s.i. Typically, during operation, these padsare maintained at a compression stress level of from 50 to about 500p.s.i. Unfortunately, the elastomer materials typically used for thepressure pad take a compression set and chemically break down whencompressed to the higher stress levels.

What is needed in the art is an improved pressure pad which maintainsuniform compression, can be utilized at pressures exceeding 2,000 p.s.i.and which is compatible with the electrochemical cell environment.

SUMMARY OF THE INVENTION

The present invention relates to a unique electrochemical cellcomprising: an anode; a cathode; a membrane disposed between said anodeand said cathode; an anode screen pack located adjacent to and inintimate contact with said anode; a cathode screen pack located adjacentto and in intimate contact with said cathode; and an electricallyconductive pressure pad located adjacent to and in intimate contact witha side of said cathode screen pack opposite said cathode.

The above discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, which are meant to be exemplary notlimiting, and wherein like elements are numbered alike in the severalFIGURES:

FIG. 1 is a schematic diagram of a prior art electrochemical cellshowing an electrochemical reaction;

FIG. 2 is a schematic diagram of a prior art electrochemical cellshowing a conventional pressure pad and its location; and

FIG. 3 is a schematic diagram of the electrochemical cell of the presentinvention showing the pressure pad and its location.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a unique pressure pad and its usewithin an electrochemical cell. This pressure pad, unlike prior artpressure pads, can be utilized on the high pressure side of theelectrochemical cell in intimate contact with the screen pack thereof,or as a replacement therefore.

The pressure pad of the present invention comprises a support materialand an electrically conductive material which is compatible with theelectrochemical cell environment. Preferably, this pressure pad has asize (especially a diameter) and geometry substantially similar to thatof the screen pack. Possible elastomeric materials include, but are notlimited to silicones, such as fluorosilicones, fluoroelastomers, such asKalrez® (commercially available from Dupont de Nemours, Wilmington,Del.), Viton® (commercially available from Dupont de Nemours), andFluorel® (commercially available from 3M, Michigan); combinations andmixtures thereof, among other elastomers, with fluoroelastomerspreferred. Possible electrically conductive materials which can beutilized in this invention include, but are not limited to, steels, suchas stainless steel, nickel, cobalt, carbon, precious metals, andrefractory metals, among others and mixtures and alloys thereof. Thetype, size, and geometry of the electrically conductive material arebased upon the need to conduct current from one side of the pressure padto the other. Consequently, any material geometry capable of conductingsuch electrical current can be utilized. Particles, cloths (woven andnonwoven), fibers (random and preformed) or other continuous pieces orstrips can be used, with, in one embodiment, fibers or other continuouspieces preferred due to the requirements of a relatively high pressureto create an electrical pad when employing particulate.

For example, strips of steel or carbon fibers can be woven intoelastomeric material to form the pressure pad. The steel strips orcarbon fibers may be interwoven with elastomeric strips or fibers, orstitched directly into an elastomeric substrate. In another example,carbon fibers and Viton cord can be woven together to form the pressurepad; where Viton cord can be directly woven into a carbon clothsubstrate, or the Viton cord and carbon fibers can be woven together.

The pressure pad is disposed in intimate contact with the high pressureflow field, which may be a screen pack. The cathode screen pack as wellas the anode screen pack can be any conventional screen capable ofsupporting the membrane, allowing the passage of hydrogen gas and water,and oxygen gas and water, respectively, and of passing electricalcurrent. Typically the screens are composed of layers of perforatedsheets or a woven mesh formed from metal or strands. These screens aretypically composed of material such as niobium, zirconium, tantalum,titanium, steels, such as stainless steel, nickel, and cobalt, amongothers and alloys thereof. The geometry of the openings in the screenscan range from ovals, circles and hexagons to diamonds and otherelongated shapes. An especially preferred screen assembly for use inelectrochemical cells is disclosed in commonly assigned U.S. patent Ser.No. 09/102,305, filed Jun. 22, 1998, now abandoned to Trent M. Molter,(herein incorporated by reference).

The screen assembly supports a membrane assembly composed of acathode/membrane/anode arrangement wherein the cathode and anode aredisposed in intimate contact with the membrane and the screenedassemblies are disposed in intimate contact with the cathode and anodeaccordingly. The membrane can be any conventional membrane including,but not limited to, proton exchange membranes including homogeneousperfluoroionomers such as Nafion® (commercially available from E.I.duPont de Nemours and Company, Wilmington, Del.), ionomer Teflon®composites such as Gore Select® (commercially available from W.L. GoreAssociates, Inc., Elkton, Md.), styrene, such as sulfonated styrene,benzene such as divinyl benzene, and mixtures thereof. Similarly, thecathode and anode electrodes can be conventional electrodes composed ofmaterials such as platinum, palladium, rhodium, carbon, gold, tantalum,tungsten, ruthenium, iridium, osmium, alloys thereof and other catalystscapable of electrolyzing water and producing hydrogen.

Referring to FIGS. 2 and 3, FIG. 2 shows a typical electrochemical cellhaving an anode 103, cathode 107, membrane 108, low pressure flow field110, high pressure flow field 112, high pressure separator plate 114,pressure pad 116. Meanwhile, FIG. 3 illustrates one embodiment of theelectrochemical cell of present invention having an anode 103, cathode107, membrane 108, low pressure flow field 110, high pressure flow field112, and an electrically conductive pressure pad 118.

In a water electrolysis cell having an active area of 0.1 square feet(ft²) and constructed in accordance with FIG. 3, for example, water at apressure of 10 p.s.i. was passed across the anode electrode by means ofa low pressure flow field chamber. A voltage of approximately 2 voltswas applied to the cell while 100 amperes of direct current (DC) weredirected through the cell. The Viton® pressure pad assembly wasmechanically loaded to 50 p.s.i., and hydrogen gas was produced at apressure of 150 p.s.i.

In another embodiment of the present invention, a high pressure fluid,such as water (under pressure up to or exceeding about 100 p.s.i., 500p.s.i., 1,000 p.s.i., or even 2,500 p.s.i.), can be introduced to thehigh pressure side of the electrochemical cell which has a high pressureflow field disposed in intimate contact with an electrically conductivepressure pad of the present invention. The water passes through the highpressure flow field, migrates from the high pressure electrode, acrossthe membrane, to the low pressure electrode where ions are formed. Theions migrate back across the membrane to the high pressure electrodewhere a second high pressure fluid is formed, such as hydrogen. The highpressure fluid then passes through the high pressure flow field.

In yet another embodiment of the present invention, a high pressurefluid (again under pressure up to or exceeding about 100 p.s.i., 500p.s.i., 1,000 p.s.i., or even 2,500 p.s.i.), can be introduced to thehigh pressure side of the electrochemical cell which has a high pressureflow field disposed in intimate contact with an electrically conductivepressure pad of the present invention. The high pressure fluid isreacted on an electrode adjacent to and in fluid communication with thehigh pressure flow field to form ions which migrate across a membrane toa low pressure electrode. At the low pressure electrode a low pressurefluid is formed. This low pressure fluid then passes through a lowpressure flow field.

Another embodiment of the present invention comprises introducing a lowpressure fluid to a low pressure flow field where the low pressure fluidreacts on an electrode adjacent to and in fluid communication with thelow pressure flow field to form ions which migrate across a membrane toa high pressure electrode. At the high pressure electrode, high pressurefluid is formed. The high pressure fluid then passes through a highpressure flow field disposed in intimate contact with the electricallyconductive pressure pad of the present invention. The pressure of thehigh pressure fluid formed can have pressures of up to 400, 1,000, or2,500 p.s.i, or greater, depending upon the system capabilities.

A further embodiment of the present invention comprises a pressure padhaving a porosity gradient. This gradient not only improves fluiddistribution to the membrane, but it also lowers the voltage requiredfor the electrochemical reaction, and provides structural integrity tothe membrane and electrode assembly, which can eliminate the need for ascreen pack. The interwoven elastomer and conductive material can belayered such that the screen pack support is enhanced or replaced, and agradient of porosity is formed. For example, layers of progressivelymore tightly woven pressure pad material can be layered to form agradient. In this example, the layered pressure pad is oriented with thegradient facing either towards or away from the membrane, and located oneither side of the membrane. In this configuration, the pressure padsserve not only as the means for ensuring the positive contact of thecell components, but also as the primary means of membrane support.

The electrochemical cell of the present invention utilizes pressure padswhich are compatible with the electrochemical cell environment, areutilized in a unique fashion by placing them on the cathode side of thecell thereby only requiring the pads to be compressed to approximately50 p.s.i., while being capable of withstanding pressures exceeding 2,000p.s.i., and even exceeding 5,000 p.s.i., with the upper pressure limitcontrolled by the system capabilities. Further advantages of the presentinvention include lower electrical resistance thereby leading to highercurrent densities, simplicity of assembly and preparation, and lowercell voltage due to elimination of screen layers and the pressure padcavity used with prior art pressure pads in order to protect them.Finally, due to the fewer parts, the electrochemical cell of the presentinvention is lower cost and has a higher reliability.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. A method for operating a high pressureelectrochemical cell, comprising: a) introducing a low pressure fluid toa low pressure flow field; b) reacting said low pressure fluid on anelectrode adjacent to and in fluid communication with the low pressureflow field to form ions which migrate across a membrane to a highpressure electrode; c) forming a high pressure fluid having a pressureexceeding 1,000 p.s.i. greater than the pressure of said low pressurefluid, at the high pressure electrode; and d) passing said high pressurefluid through a high pressure flow field disposed in intimate contactwith an electrically conductive pressure pad.
 2. A method for operatinga high pressure electrochemical cell as in claim 1, wherein saidpressure pad maintains substantially uniform pressure on said highpressure flow field.
 3. A method for operating a high pressureelectrochemical cell as in claim 1, wherein said pressure pad iscompatible with the electrochemical cell and can function in pressuresexceeding 2,000 p.s.i.
 4. A method for operating a high pressureelectrochemical cell as in claim 1, wherein said pressure pad comprisesa support material and an electrically conductive material.
 5. A methodfor operating a high pressure electrochemical cell as in claim 4,wherein said support material is a fluorosilicone, fluoroelastomer, ormixture thereof.
 6. A method for operating a high pressureelectrochemical cell as in claim 4, wherein said electrically conductivematerial is steel.
 7. A method for operating a high pressureelectrochemical cell as in claim 4, wherein said electrically conductivematerial is a stainless steel, a nickel, cobalt, carbon, refractorymetal, precious metal, or a mixture or alloy thereof.
 8. A method foroperating a high pressure electrochemical cell as in claim 1, whereinsaid low pressure flow field is a low pressure screen pack locatedadjacent to and in intimate contact with said low pressure sideelectrode.
 9. A method for operating a high pressure electrochemicalcell as in claim 1, wherein said high pressure low field is a highpressure screen pack located adjacent to and in intimate contact withsaid high pressure side electrode.
 10. A method for operating a highpressure electrochemical cell as in claim 1, wherein said pressure padis compatible with the electrochemical cell and can function inpressures exceeding 5,000 p.s.i.
 11. A method for operating a highpressure electrochemical cell as in claim 1, wherein said pressure padcomprises one or more layers of conductive material interwoven with anelastomer, said layers each having a porosity.
 12. A method foroperating a high pressure electrochemical cell as in claim 11, whereinsaid conductive material is carbon.
 13. A method for operating a highpressure electrochemical cell as in claim 11, wherein said elastomer isa fluorosilicone, fluoroelastomer, or combinations thereof.
 14. A methodfor operating a high pressure electrochemical cell as in claim 11,wherein said pressure pad has a gradient of porosity.
 15. A method foroperating a high pressure electrochemical cell as in claim 14, whereinsaid layers of differing porosity form said gradient.
 16. A method foroperating a high pressure electrochemical cell as in claim 11, whereinthe high pressure field operates at above 2000 p.s.i.
 17. A method foroperating a high pressure electrochemical cell, comprising: a)introducing a high pressure fluid having a pressure exceeding about 100p.s.i. to a high pressure flow field disposed in intimate contact withan electrically conductive pressure pad; b) reacting said high pressurefluid on an electrode adjacent to and in fluid communication with thehigh pressure flow field to form ions which migrate across a membrane toa low pressure electrode; c) forming a low pressure fluid at the lowpressure electrode; and d) passing said low pressure fluid through a lowpressure flow field.
 18. A method for operating a high pressureelectrochemical cell as in claim 17, wherein said pressure exceeds about500 p.s.i.
 19. A method for operating a high pressure electrochemicalcell as in claim 17, wherein said pressure exceeds about 1,000 p.s.i.20. A method for operating a high pressure electrochemical cell as inclaim 17, wherein said pressure pad comprises one or more layers ofconductive material interwoven with an elastomer, said layers eachhaving a porosity.
 21. A method for operating a high pressureelectrochemical cell as in claim 20, wherein said conductive material iscarbon.
 22. A method for operating a high pressure electrochemical cellas in claim 20, wherein said elastomer is a fluorosilicone,fluoroelastomer materials, or combinations thereof.
 23. A method foroperating a high pressure electrochemical cell as in claim 20, whereinsaid pressure pad has a gradient of porosity.
 24. A method for operatinga high pressure electrochemical cell as in claim 23, wherein said layersof differing porosity form said gradient.
 25. A method for operating ahigh pressure electrochemical cell as in claim 20, wherein the highpressure field operates at above 2000 p.s.i.
 26. A method for operatinga high pressure electrochemical cell, comprising: a) introducing a firsthigh pressure fluid having a pressure exceeding about 100 p.s.i. to ahigh pressure flow field disposed in intimate contact with anelectrically conductive pressure pad; b) migrating at least a portion ofsaid first high pressure fluid from a high pressure electrode, across amembrane, to a low pressure electrode; b) reacting said first highpressure fluid on said low pressure electrode to form ions which migrateacross said membrane to said high pressure electrode; c) forming asecond high pressure fluid at said high pressure electrode; and d)passing said second high pressure fluid through said high pressure flowfield.
 27. A method for operating a high pressure electrochemical cellas in claim 26, wherein said pressure pad comprises one or more layersof conductive material interwoven with an elastomer, said layers eachhaving a porosity.
 28. A method for operating a high pressureelectrochemical cell as in claim 27, wherein said conductive material iscarbon.
 29. A met hod for operating a high pressure electrochemical cellas in claim 27, wherein said elastomer is a fluorosilicone,fluoroelastomer, or combinations thereof.
 30. A method for operating ahigh pressure electrochemical cell as in claim 27, wherein said pressurepad has a gradient of porosity.
 31. A method for operating a highpressure electrochemical cell as in claim 30, wherein said layers ofdiffering porosity form said gradient.
 32. A method for operating a highpressure electrochemical cell as in claim 27, wherein the high pressurefield operates at above 2000 p.s.i.
 33. An electrochemical cell pressurepad, comprising one or more layers of conductive material interwovenwith an elastomer, said layers each having a porosity.
 34. Anelectrochemical cell pressure pad as in claim 33, wherein saidconductive material is carbon.
 35. An electrochemical cell pressure padas in claim 33, wherein said elastomer is a fluorosilicone,fluoroelastomer, or combinations thereof.
 36. An electrochemical cellpressure pad as in claim 33, wherein said pressure pad has a gradient ofporosity.
 37. An electrochemical cell pressure pad as in claim 36,wherein said layers of differing porosity form said gradient.
 38. Anelectrochemical cell pressure pad as in claim 36, wherein the highpressure field operates at above 2000 p.s.i.